A Washington Square Park drug dealer’s stickup plan went bust – after the young would-be buyer he stiffed boldly ratted him out to cops, authorities said.

A teenager told cops that he approached Derrick Gary, 49, on Sixth Avenue and West Fourth Street on Friday afternoon to buy pot. He handed over $10, and Gary allegedly started to take off.

When the victim asked for his money back, Gary whipped out a knife and sneered, “I’m not giving you the money back. I’m keeping it,” sources said. The victim called cops, and Gary was picked up a few blocks away.

He was charged with robbery, criminal possession of a weapon and criminal possession of marijuana, according the Manhattan District Attorney’s Office.

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A thug looking for his piece of the pie has been nabbed for holding up a worker at an Upper East Side pizzeria, authorities said yesterday.

Douglas Welsh, 34, approached the 20-year-old victim at the Papa John’s on Second Avenue at East 91st Street on May 26, officials said. He put his hand to his waist and allegedly said, “I have a gun under my shirt. Give me the f – – -ing money. Don’t play around.”

The victim handed over $150, and Welsh fled the scene. The robbery was caught on video surveillance, and cops received a tip that Welsh was the perp. He was busted about a week later.

Welsh was charged with robbery, according to a spokeswoman for the DA.

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Police are asking for the public’s assistance in finding this elderly man , who disappeared from his East Harlem home.

Enrique Picart, 77, who suffers from Alzheimer’s disease, was last seen leaving his East 118 Street home at 9:30 a.m. Saturday. He is 5-foot-10 and 150 pounds, and was wearing a dark-green polo shirt, gray pants and brown shoes.

Brooklyn

A man who told cops he was the president of the United States went on a rampage in Bushwick, hurling a brick at a car, spraying a fire extinguisher into traffic and scrapping with officers, authorities said yesterday.

Joseph Burgess, 46, was spotted stealing the extinguisher from a gas station and spraying it into traffic after tossing a brick at the victim’s car on Bushwick Avenue near Stewart Street at 2:40 p.m. Sunday, sources said.

When officers arrived on the scene, they spotted Burgess running around like a maniac and screaming. Cops pursued him, and the suspect allegedly ran into traffic and jumped on the hood of a car. As they caught up to him, Burgess allegedly whipped a set of keys at one of the cops and flailed his arms wildly.

Two men were collared after cops found a gun in their car when the gate in an E-ZPass lane wouldn’t open for them at a Brooklyn Battery Tunnel tollbooth, authorities said.

The driver, Frank Turner, 37, and passenger, William Emanuel, 39, were in a 2007 Nissan Pathfinder heading for Manhattan on Sunday evening. The E-ZPass tag on their windshield was scanned, and when the barrier didn’t go up, cops pulled them over. Investigators found that Turner had a suspended license – and a handgun in the back seat.

Staten Island

A teenage perv was busted for groping an 11-year-old girl in a Port Richmond playground, authorities said yesterday.

Erick Flores, who is 18 and homeless, sat down next to the young victim on a bench in the park on Willowbrook Road at Houston Street just after 4 p.m. Sunday, sources said.

He then asked the girl if he could give her a massage or if she could massage him, and then ran his hand down her leg, according to authorities.

The girl’s older brother witnessed the sickening episode and came to his sister’s rescue, holding the suspect until cops arrived.

He was charged with endangering the welfare of a child and harassment, a spokesman for the DA said.

The Bronx

A man was found shot to death on a street in the Pelham Parkway section, police said yesterday.

The unidentified 30-year-old victim was found with a gunshot wound to the chest in front of 2181 Barnes Ave. near Pelham Parkway just before 11 p.m. Sunday. He was pronounced dead at the scene.